The invention relates to a quadrupole mass spectrometer.
The quadrupole mass spectrometer has a structure, wherein a sample molecule or ion is ionized in an ion source, and the ions produced therein are introduced into a quadrupole filter to allow only the ions with a predetermined mass number to selectively pass therethrough to thereby detect the passed ions at a detector. Since the mass number of the ions passing through the quadrupole filter is changed according to a voltage applied thereto, the mass number of the ions reaching the detector can be scanned or controlled over a predetermined mass region by scanning the voltage applied thereto. This is, what is called, a scanning by the quadrupole mass spectrometer.
In a gas chromatograph mass spectrometer (hereinafter referred to as xe2x80x9cGC/MSxe2x80x9d) wherein a gas chromatograph (hereinafter referred to as xe2x80x9cGCxe2x80x9d) is connected to a preceding step of a mass spectrometer (hereinafter referred to as xe2x80x9cMSxe2x80x9d), since sample component molecules separated during movement together with a carrier gas in a column of the GC are introduced into an ion source of the MS, a fairly large amount of carrier gas molecules when compared with a specified component, is supplied to the ion source. Also, in case impurity components contained in a sample gas including a main component are analyzed by the MS, a fairly large amount of the main component molecules as compared with the specified component, i.e. impurity components, is supplied to the ion source.
As a matter of course, the carrier gas molecules and the main component molecules are ionized in the ion source. A quantity of these ions is extremely large as compared with those produced from the molecules of the specific component. In case such a large amount of ions passes through the quadrupole filter and reaches the detector in a short time, the detector is saturated with these ions to thereby deteriorate its characteristics. In order to prevent the saturation, a sensitivity of the detector can be lowered beforehand. However, in case the specific component is a very small amount, when the sensitivity of the detector is lowered, the specific component itself can not be detected or a detecting accuracy becomes poor, which deteriorates an analysis.
Therefore, when a scanning or operation is carried out by a conventional mass spectrometer, a mass region except for a mass number corresponding to the undesired excessive ions has been set. More specifically, for example, in case the mass number of the excessive ions is m1 and a mass region of m2xe2x88x92m3 (m2xe2x89xa6m1xe2x89xa6m3) is scanned, it has been required to carry out two scannings by setting two mass regions, such as m2xe2x88x92m1 (not including m1) and m1xe2x88x92m3 (not including m1). Therefore, the scanning work becomes complicated and a working efficiency is poor.
To solve the above problems, the present invention has been made and an object of the invention is to provide a quadrupole mass spectrometer, which can eliminate the bad influence of excessive ions to a detector without setting, in scanning, a mass region including a mass number corresponding to the excessive ions unnecessary for analysis.
Another object of the invention is to provide a quadrupole mass spectrometer as stated above, wherein an analysis can be carried out with high sensitivity and accuracy.
Further objects and advantages of the invention will be apparent from the following description of the invention.
In order to attain the above objects, a quadrupole mass spectrometer of the invention includes an ion source for ionizing a sample molecule; a quadrupole filter for selectively passing ions with a specific mass number among the ions generated in the ion source; and a detector for detecting the ions passing through the quadrupole filter. In the quadrupole mass spectrometer, when a voltage to be applied to the quadrupole filter is scanned or arranged to successively change a mass number of the ions passing through the quadrupole filter, a DC-type electric field is temporarily changed at a space between the ion source and the quadrupole filter or a part of the space according to a timing of passage of the ions having the specific mass number set in advance and passing through the quadrupole filter.
Normally, an ion lens for converging the ions is disposed between the ion source and the quadrupole filter. In this case, in the quadrupole mass spectrometer of the invention, a DC potential may be relatively changed between the ion lens and the ion source, the quadrupole filter and the ion source, or the quadrupole filter and the ion lens.
In the quadrupole mass spectrometer of the present invention, when the DC-type or linear electric field is temporarily changed at the time of scanning as described above, the ions to be introduced into the quadrupole filter from the ion source lose their kinetic energy in the middle of their path to suddenly decelerate their flight speed, change their converging state, or are not drawn out from the ion source. Therefore, when the ions having the specified mass number can pass through the quadrupole filter, all the ions including the ions having the specified mass number are not introduced into the quadrupole filter, or the quantity of the introduced ions, if introduced, is extremely reduced. Also, even if the ions again start flying toward the quadrupole filter and reach the quadrupole filter after the electric field returns to the original state, at the time point, the quadrupole filter does not have the electric field for allowing the ions with the specific mass number to pass therethrough to thereby diffuse the ions in the middle of passage through an inner space of the quadrupole filter, so that the ions do not reach the detector.
Therefore, according to the quadrupole mass spectrometer of the present invention, even if the mass region is set to include a mass number corresponding to undesired excessive ions of molecules of a carrier gas or an unnecessary main component introduced into the ion source simultaneously with introduction of a specific component, the undesired excessive ions can be prevented from reaching the detector. Thus, it is not necessity to set the mass region to exclude the mass number corresponding to the undesired excessive ions as in the conventional mass spectrometer, so that a scanning or operation can be carried out effectively. Also, since there is no risk of saturating the detector with the excessive ions, it is not necessary to lower a sensitivity of the detector, so that an analysis with high sensitivity and accuracy can be performed.